This invention relates to a planetary transmission. It relates more particularly to a transmission of this type which is particularly efficient and relatively easy and inexpensive to manufacture in quantity.
Planetary transmissions are, of course, quite old. They are used to promote transmission of power while involving relatively small bulk and but a few parts which desirably move at relatively slow speeds. The gearing preferred for such purposes comprises paired orbiting and nonorbiting gears in which substantially continuous rolling contacts occur between many gear teeth which are simultaneously in torque-transmitting engagements. Examples of such transmissions are disclosed in the following patents:
U.S. Pat. No. Re. 24,288 Nanni, PA1 U.S. Pat. No. 3,056,315 Mros, PA1 U.S. Pat. No. 3,073,184 Braren, PA1 U.S. Pat. No. 3,192,799 Pamplin, PA1 U.S. Pat. No. 3,985,047 Therkelsen, PA1 U.S. Pat. No. 3,998,112 Pierrat, PA1 U.S. Pat. No. 4,023,440 Kennington, PA1 U.S. Pat. No. 542,206 Fed. Rep. of Germany.
Of particular relevance for purposes of this invention are U.S. Pat. Nos. 3,998,112 and 4,117,746 which are owned by the assignee of the present application.
The prior transmissions of this general type employ internal orbiting gears having teeth or lobes of epitrochoidal curvature which cooperate with a surrounding external nonorbiting gear whose teeth are in the form of cylindrical pins or rollers and exceed by one the number of teeth in the internal orbiting gear. In those transmissions, there are at least two orbiting and nonorbiting gear pairs. Also the oribiting gears are interchangeable and have the same diameter and mass eccentricities so that they remain matched with counterweighting despite their different numbers of teeth. The cooperating nonorbiting gears are likewise all of the same diameter and have their teeth conveniently disposed at one predetermined radial distance from a central axis.
Prior transmissions of that general type are disadvantaged in that they are relatively expensive to manufacture. The main reasons for this are that the internal orbiting gears have to be machined from strong metal stock such as tool steel because of the very high Hertzian stresses on the peripheries of the externally lobed orbiting gears. Also close tolerances must be maintained between a central shaft-receiving bore or barrel in each orbiting gear and the peripheral gear teeth or lobes which because of the relatively small radii of curvature of the lobe surfaces of the oribiting gears, are relatively steeply pitched and therefore difficult to machine with the required accuracy.
Further, since the nonorbiting gears in the prior transmissions have teeth in the forms of fixed circular arrays of pins spaced radially outward from the gear axes, maintenance of sufficiently high tolerances between those pins and the bore or barrel through the orbiting gears are equally hard to maintain. The result is that the prior transmissions have a tolerance stack-up problem which renders their output motions not as smooth as might be desired. This is particularly so in the case of transmissions subjected to a heavy load which tends to deflect the orbiting gears to the opposite sides of the transmission housing to the extent that their "nondriving" lobes actually interfere with pins on the opposite sides of the nonorbiting gears.
In addition, the radii of curvature of the lobe surfaces of the orbiting gears in the prior drives of this type are relatively small. Therefore, their Hertzian load carrying capacities and hence their torque load capacities are not as high as might be desired. Also the prior transmissions employing pins as the nonorbiting gear teeth are not as efficient as might be claimed or wished. This is because the pins, being rotatable but peripherally restrained in their sockets, are insufficiently lubricated so that a considerable amount of sliding friction exists between the orbiting and nonorbiting gears. Of course, this also promotes excessive parts wear and subsequently, increases in roughness of operation, and noise, a reduced operating life, and a deterioration in efficiency.